ARTICLE pubs.acs.org/IECR
Degradation of Tributyl Phosphate Using Nanopowders of Iron and IronNickel under the Influence of a Static Magnetic Field Ritu D. Ambashta,*,†,‡ Eveliina Repo,§ and Mika Sillanp€a€a§ †
Backend Technology Development Division, Bhabha Atomic Research Centre, Mumbai-400085, India Laboratory of Applied Environmental Chemistry, University of Eastern Finland, Patteristonkatu 1, FI-50100 Mikkeli, Finland § Laboratory of Green Chemistry, Lappeenranta University of Technology, Patteristonkatu 1, FI-50100 Mikkeli, Finland ‡
ABSTRACT: A novel method for the degradation of tributyl phosphate was investigated through the use of nanopowders of iron and ironnickel. A combination of magnetic field and sonication was used to induce degradation. Using a tributyl phosphate concentration of 300 ppm at pH 4, the Fenton reaction was initiated in the presence of sonication and a reduction in organic carbon content of up to 50% could be created under an induced magnetic field of 0.7 T. The magnetic properties of powders after sonication treatment, in the presence and absence of a magnetic field, were observed to be different. Different degradation mechanisms were proposed under the influence of varying induced magnetic field for iron and ironnickel.
1.0. INTRODUCTION Trialkyl phosphate esters are a class of anthropogenic organics commonly found in surface waters, because of their application as flame retardants, plasticizers, and solvents.1 In nuclear reprocessing plants, tributyl phosphate (hereafter referenced as TBP) is commonly employed for the recovery of uranium and plutonium from spent fuel.2 The presence of this ester in mixed wastes does not allow one-step decontamination of radionuclides using exchange, precipitation, or extraction. The strategy for treatment requires two-stage treatment, viz, organic contaminant removal followed by the removal of other contaminants. In both soil and water, TBP is expected to adsorb to sediments or particulate matter and biodegrade.3 Being carcinogenic in nature, its discharge into nature must be controlled. However, it has been demonstrated that there is low anaerobic degradation of alkylphosphate by nature.4 Several methods of mineralization have been experimentally demonstrated. Mixed microbial culture and biomaterial sources (for example, Serratia odorifera) have been used for the degradation of TBP.5,6 The other methods of degradation include chemical methods,7 acidic hydrolysis and precipitation,8 photo-Fenton,3,9,10 membrane anodic Fenton,11 combination of supercritical carbon dioxide extraction (at 90 °C and 325 atm), and heterogeneous photocatalytic oxidation.1217 Nanoscale photocatalysts help in enhancing the oxidizing power of Fenton-type systems. However, after degradation, the separation of the sludge of nanoscale catalyst from the mineralized solution poses another challenge to secondary waste management. Magnetism could enable an efficient separation of the sludge after treatment. Therefore, magnetic photocatalysts have been employed for degradation at the photoreactor and sludge collection at the magnetic separator.1821 However, this requires operation of two separate reactor designs: one for the photodegradation and another for the magnetic separation. The present study is aimed at a common technology application for degradation and separation of organic compounds from an aqueous medium. Fenton reactions have been studied under magnetic field, using iron in solution form, to understand radical r 2011 American Chemical Society
behavior.22,23 It was found that, under optimal Fenton reaction conditions, with the assistance of magnetic field, the degradation rate of target organic compound could be accelerated. The corrosion reaction using metallic or zerovalent iron has been exploited for the degradation of several organics.2428 Magnetically assisted chemical processing has been demonstrated for several applications in separation sciences, synthesis methods, and catalytic methods.2935 It has been observed that, under the influence of magnetic field, the zerovalent iron reactivity would be reduced, because of agglomeration, and, therefore, the purpose of small-size reactivity is lost.36 The method of sonication has been applied in the degradation of organic compounds at different ultrasound frequencies.3740 It is a good dispersion source as well. Combining the sonication and magnetic process, in the present study, zerovalent iron and ironnickel nanoparticles were studied for the degradation of TBP.
2.0. EXPERIMENTS 2.1. Materials and Reagents. Nanopowders of iron [FE-M03M-NP.100N] and ironnickel [FE-NI-017-NP.100N] were received from M/s American Elements. Tributyl phosphate (TBP, synthesis-grade), which was procured from M/s Merck, and analytical-grade hydrochloric acid (HCl) were used for the experiments. 2.2. Catalytic Experiments. A DC-powered electromagnet (procured from M/s SVS Lab, Inc.) was used in the experiments. A stock solution of 300 mg/L TBP was prepared in a HCl solution (pH 4). The samples of iron and nickel were dispersed under sonication, using a Branson Model 2510 Sonicator. All reactions were carried out in a test tube with an inner diameter Received: October 19, 2010 Accepted: September 6, 2011 Revised: August 8, 2011 Published: September 07, 2011 11771
dx.doi.org/10.1021/ie102121e | Ind. Eng. Chem. Res. 2011, 50, 11771–11777
Industrial & Engineering Chemistry Research
Figure 1. Nitrogen adsorptiondesorption isothermal curves of iron and ironnickel powders, along with the pore size distribution. (Legend: (0) iron and (O) ironnickel.)
(ID) of 7 mm and a height of 300 mm). A study on variation with time and electromagnetic field was carried out for the nanopowders of iron and ironnickel. For the variation-in-time study, the solution-to-powder ratio was maintained at 1000:1 (volume [mL] by weight [g]) and iron and ironnickel were sonicated under induced magnetic field of 0.3 T for 30, 60, and 90 min respectively. For a variation in electromagnetic field study, the solution-to-powder ratio was maintained at 1000:1 (volume [mL] by weight [g]) and iron and ironnickel were sonicated for 60 min under induced magnetic fields of 0.1, 0.3, 0.5, and 0.7 T, respectively. 2.3. Characterization of Materials. A PANalytical X’Pert Pro MPD-model X-ray diffractometer was used to investigate the crystalline nature of the powders. A Hitachi Model S-4800 ultrahigh resolution scanning electron microscopy (SEM) system was used to understand the morphology of powders. The total organic carbon (hereafter denoted as TOC) content analysis was carried out using a Shimadzu Model TOC-VCPH analyzer. The magnetization data was evaluated using a Quantum Design SQUID-MPMS XL magnetometer. Surface area, particle size, and pore diameter were evaluated using Autosorb 1 equipment (QuantaChrome Instruments). The elements in the solution were analyzed using Thermo Electron Corporation Model iCAP 6000 Series inductively coupled plasma (ICP) spectrometer. The samples were diluted in 2% nitric acid (HNO3) for each measurement. The standards were also prepared in 2% nitric acid (HNO3).
3.0. RESULTS AND DISCUSSIONS 3.1. Powder Characterization. The surface area (as determined using the BrunauerEmmettTeller (BET) method) and pore structure of iron and ironnickel were determined from nitrogen isothermal analyses, as shown in Figure 1. Using multipoint BET, BarrettJoynerHalenda (BJH) method cumulative desorption, and DollimoreHeal (DH) method cumulative desorption methods, the average surface area of iron was evaluated to be 10 m2/g, whereas the average surface area of ironnickel was 27 m2/g. The average pore diameters were 337
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Figure 2. X-ray diffraction (XRD) pattern of nanopowder of iron and nickel before treatment and after sonication treatment under an induced field of 0.3 T for 60 min.
and 753 Å for iron and ironnickel, respectively, and the total pore volumes were 0.086 and 0.42 cm3/g, respectively. A stock solution of 300 ppm TBP was prepared in a HCl solution (pH 4). Maintaining the solution-to-powder ratio at 1000:1 (volume [mL] by weight [g]), iron and ironnickel were sonicated under an induced magnetic field of 0.3 T for 1 h. The temperature of the solution was increased in the process by 58 °C. The comparison of X-ray diffraction (XRD) patterns and the morphology of iron and ironnickel sonicated for 1 h under a magnetic field of 0.3 T is shown in Figure 2. The XRD pattern of untreated iron and ironnickel showed the presence of body-centered cubic iron (PDF File Card No. 006-0696) and hexagonal close-packed FeNi (PDF File Card No. 023-0297). Sonication under the influence of magnetic field introduced phases of iron(II) iron(III) mixed oxide (magnetite) (PDF File Card No. 19-629) in the case of treated iron. In the case of ironnickel, the mixed oxide is a combination of inverse spinel nickel ferrite and other iron/nickel oxides. The SEM images of the treated and untreated iron and ironnickel are shown in Figure 3. The images suggested that, after treatment, the spherical shape of the powders transformed to a rod shape. The particles are dispersed and are aligned along the direction of the magnetic field; therefore, a shape transformation occurs. The energy-dispersion X-ray (EDX) spectroscopy data over four randomly selected locations on iron and ironnickel are summarized in Table 1. The iron-to-oxygen ratio was 2.43 for untreated iron, which was reduced to 0.57 for the treated iron with the increase in oxygen content. In the case of ironnickel nanopowders, for the treated ironnickel, the ironto-oxygen ratio is reduced from 4.12 to 1.8 and the nickel-tooxygen ratio is reduced from 3.1 to 1.6. The magnetization measurement on untreated and treated iron and ironnickel is shown in Figure 4. Using a plot of magnetization (M) vs the inverse induced field (1/B), with B f ∞, the saturation magnetization (at the intercept) value of iron was evaluated as being 187 and 203 emu/g for the untreated and treated samples. The values for ironnickel were 11772
dx.doi.org/10.1021/ie102121e |Ind. Eng. Chem. Res. 2011, 50, 11771–11777
Industrial & Engineering Chemistry Research
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Figure 3. Scanning electron microscopy (SEM) image of nanopowders of iron and nickel before and after sonication treatment under an induced field of 0.3 T for 60 min.
Alternatively,
Table 1. Elemental Composition, As Observed by EDX Composition (at. %) sample
O
Fe
Ni
untreated iron
29.16
70.84
treated iron untreated ironnickel
63.55 12.17
36.45 50.21
37.63
treated ironnickel
22.39
41.97
35.64
155 and 130 emu/g for the untreated and treated samples, respectively. The oxidation of iron and ironnickel led to the formation of iron(II), iron(III), and nickel(II) mixed oxides. The mixed-valency iron oxides are ferrimagnetic in nature while ironnickel oxides can translate to ferrimagnetic or paramagnetic compound formation, as was confirmed by XRD. Paramagnets, being weaker magnets, caused a reduction in magnetization of ironnickel and an enhancement of magnetization in iron. 3.2. Degradation Behavior with Time. Using TOC data, the plot of final organic carbon, with respect to the initial organic carbon amount, versus time was obtained as shown in Figure 5. The standard deviation evaluated using TOC standards was ∼4%. The data suggested an increase in degradation with time. TBP degraded with an increase in pH from 4 to 4.5. Iron or ironnickel degrades TBP, according to the mechanism þ
Fe þ O2 þ 2H f Fe 0
H2 O2 f H 2 O þ
1 O2 2
2þ
þ H2 O2
Fe2þ þ H2 O2 f FeOH2þ þ OH 3
ð4Þ
ðC4 H9 OÞ3 PO þ OH 3 f ðC4 H9 OÞ2 ð 3 C4 H8 OÞPO þ H2 O
ð5Þ
The radical, after multiple stages, degrades to the mineralized forms of reaction 3. One proposed route of mineralization is described as follows:41 C3 H7 CH 3 OPOðOC4 H9 Þ2 þ O2 f C3 H7 CHOO 3 OPOðOC4 H9 Þ2
ð6Þ
The alkoxy radical undergoes hydrogen atom transposition to give a carbon radical with an hydroxybutyl chain: C3 H7 CHO 3 OPOðOC4 H9 Þ2 f C3 H7 CHOHPOOCH 3 C3 H7 ðOC4 H9 Þ
ð7Þ This carbon radical undergoes the same process (Bu = C4H9, Pr = C3H7) twice, followed by hydrogen transfer to an oxygen molecule, to give bis-hydroxybutyl-oxobutylphosphate: PrCHOHOPOOCH 3 PrðOBuÞ þ O2 f PrCHOHPOOCHOO 3 PrðOBuÞ
ð8Þ
PrCHOHOPOOCHO 3 PrðOBuÞ f ðPrCHOHOÞ2 POOCH 3 Pr
ð1Þ
ð9Þ ðPrCHOHOÞ2 POOCH 3 C3 H7 f ðPrCHOHOÞ2 POOCHOO 3 Pr
ð2Þ
ð10Þ
ðC4 H9 OÞ3 PO þ 18O2 f 12CO2 þ 3Hþ þ PO3 4 þ 12H2 O
ðPrCHOHOÞ2 POOCHO 3 Pr þ O2 f ðPrCHOHOÞ2 POOCOPr
ð3Þ
þ HO23 11773
ð11Þ
dx.doi.org/10.1021/ie102121e |Ind. Eng. Chem. Res. 2011, 50, 11771–11777
Industrial & Engineering Chemistry Research
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Figure 4. Plot of magnetization versus induced field of nanopowder of iron and nickel before treatment and after sonication treatment under an induced field of 0.3 T for 60 min. Inset shows a plot of Magnetization versus the inverse of the induced magnetic field. The intercept is related to the saturation magnetization.
This last compound reacts with water to give phosphoric acid (which is later extracted in the aqueous phase): ðPrCHOHOÞ2 POOCOPr þ 3H2 O f H3 PO4 þ 2PrCHO þ PrCOOH
ð12Þ
The aldehyde and carboxylic acid are also further amenable to mineralization. Iron or nickel oxidized to release Fe or Ni ions that quenched the phosphate and therefore enhanced the reaction in the forward direction. Further iron and nickel from the solid surface were available as fresh sites for further oxidation and the release of ions. These also added to the formation of additional H2O2. M þ O2 þ 2Hþ f M2þ þ H2 O2
ð13Þ
3.4. Degradation Behavior under Different Induced Fields. Using TOC data, the plot of final organic carbon, with
magnetic field was obtained as shown in Figure 6. In the absence of magnetic field, under sonication conditions, iron (C/C0 = 0.7) degraded a greater amount of TBP than ironnickel (C/C0 = 0.74). Although this value difference was
